3b3    SOD 


8061  '12  KVP 
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THE  EFFECT  OF   CERTAIN   DISSOLVED 

SUBSTANCES  ON  THE  INFRA-RED 

ABSORPTION  OF  WATER 


By 
J.  R.  COLLINS 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE    GRADUATE  SCHOOL  OF   CORNELL 

UNIVERSITY  IN  PARTIAL  FULFILLMENT  OF  THE    REQUIREMENTS 

FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 


Reprinted  from  THE  PHYSICAL  REVIEW,  N.  S.,  Vol.  XX.,  No.  5,  November,  1922 


THE  EFFECT  OF   CERTAIN    DISSOLVED 

SUBSTANCES  ON  THE  INFRA-RED 

ABSORPTION  OF  WATER 


By 
J.  R.  COLLINS 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE    GRADUATE  SCHOOL  OF   CORNELL 

UNIVERSITY  IN  PARTIAL  FULFILLMENT  OF  THE    REQUIREMENTS 

FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 


Reprinted  from  THE  PHYSICAL  REVIEW,  N.  S.,  Vol.  XX.,  No.  5,  November,  1922 


A 


THE  EFFECT  OF  CERTAIN  DISSOLVED  SUBSTANCES 
ON  THE  INFRA-RED  ABSORPTION  OF  WATER. 


50780V 


THE    EFFECT   OF    CERTAIN    DISSOLVED    SUBSTANCES   ON 
THE   INFRA-RED   ABSORPTION   OF  WATER. 

BY  J.  R.  COLLINS. 
SYNOPSIS. 

Infra-red  absorption  spectra  of  aqueous  solutions  of  sixteen  inorganic  compounds 
from  0.8  to  2.3  /*,  have  been  determined  to  see  how  the  disvsolved  substances  affect 
the  absorption  of  the  water.  Care  was  taken  to  eliminate  stray  light  by  using  two 
constant  deviation  glass  spectroscopes  in  series.  Readings  were  taken  alternately 
with  a  cell  containing  a  solution  and  with  one  containing  water.  Curves  were  thus 
obtained  for  the  alkali  hydroxides,  five  chlorides  (Al,  Ca,  Mg,  Na,  Sr),  five  nitrates 
(Ag,  Al,  Mg,  NHt,  Zri)  and  three  sulphates  (Ag,  Na,  Zri).  (Figs.  3-16).  All 
solutes  decreased  the  absorption  in  the  water  band  at  i-44ju  and  probably  also  in 
the  band  at  2  /*,  whereas  all  excepting  Ah  (SO 4)3,  ZnSC>4  and  the  hydroxides  increased 
the  absorption  in  the  bands  at  0.97  and  1.2  ju.  These  results  do  not  agree  with  the 
solvate  theory  which  ascribes  the  effect  to  the  formation  of  hydrates,  since  some 
non-hydrating  compounds  decreased  the  absorption  at  1.44  At. 

Absorption  of  water,  from  0.8  to  2.3/4,  was  measured  by  using  cells  of  different 
known  thicknesses.  The  wave-lengths  of  maximum  absorption  were  found  to  be  0.97, 
1.20,  1.44,  and  2.OOJU,  and  the  corresponding  coefficients  came  out  0.448,  1.220,  29.4, 
and  103,  respectively.  Such  difference  as  there  is  between  these  results  and  those 
previously  obtained  may  be  due  to  elimination  of  stray  light.  The  fact  that  in  the 
absorption  spectrum  of  water  vapor  the  bands  at  1.44  and  2  /JL  are  stronger  and 
the  other  two  bands  weaker  than  for  liquid  water  suggests  that  the  former  two 
bands  are  associated  with  a  different  kind  of  molecule  than  the  latter  two  bands. 
If  so,  the  effect  of  a  dissolved  substance  on  absorption  may  be  due  to  a  change  in  the 
relative  number  of  these  kinds  of  molecules  produced  by  the  presence  of  the  substance. 

INTRODUCTION. 

TN  their  study  of  infra-red  absorption  spectra  of  aqueous  solutions, 
-^  Guy,  Schaeffer,  and  Jones1  found  that,  in  some  cases,  the  solutions 
were  more  transparent  than  the  water  present  would  be  if  there  were 
no  dissolved  substance  in  it.  Schaeffer,  Paulus,  arid  Jones2  studied  this 
effect  of  dissolved  substances  on  the  absorption  of  water  further  and 
found  that  the  change  in  the  absorption  of  the  water  was  generally 
accompanied  by  a  slight  shift  in  the  position  of  the  absorption  band 
toward  the  longer  wave-lengths.  Grantham3  found  that  the  alkaline 
hydroxides  dissolved  in  water  caused  a  decrease  in  the  absorption  of 
water  at  one  absorption  band,  the  effect  due  to  these  substances  being 
greater  than  that  due  to  the  inorganic  salts  studied  by  the  previous 
investigators.  No  conclusion  can  be  drawn  as  to  a  shift  in  the  position 

1  Phys.  Zeitschr.,  14,  p.  278  (1913). 

2  Phys.  Zeitschr.,  15,  p.  447  (1914). 
8  PHYS.  REV.,  18,  p.  339  (1921). 


VoL.XX.J  INFRA-RED   ABSORPTION  OF    WATER.  487 

of  the  absorption  band  caused  by  the  alkaline  hydroxides,  since  the 
hydroxides  have  an  absorption  band  which  overlaps  that  of  water,  thus 
making  it  impossible  to  distinguish  the  exact  form  of  either  absorption 
band.  Further  evidence  of  a  shift  in  the  position  of  an  absorption  band 
of  water  by  dissolved  substances  was  obtained  by  Angstrom,1  who 
measured  the  reflection  of  infra-red  radiation  from  the  surfaces  of  solu- 
tions. A  shift  in  the  positions  of  maximum  and  minimum  reflection  is 
interpreted  as  a  shift  in  the  position  of  the  absorption  band.  No  informa- 
tion was  obtained  as  to  changes  in  the  intensity  of  absorption  of  the  water. 

The  most  intense  absorption  bands  of  water  occur  at  about  3  /z  and 
6  fjL.  Nearer  the  visible  spectrum  there  are  four  much  less  intense  bands 
which  differ  greatly  among  themselves  in  intensity.  These  bands  were 
located  by  earlier  observers  at  about  i.Oju,  1.2 //,  1.57-1  and  2.0/1,  this 
order  being  also  one  of  increasing  intensity.  Jones  and  his  co-workers 
studied  the  effect  of  dissolved  substances  on  the  absorption  of  water  in 
the  spectral  region  including  the  first  two  of  these  bands.  Grantham's 
results  on  the  effect  of  the  alkaline  hydroxides  are  confined  to  the 
absorption  band  at  1.5  ju.  Angstrom  worked  in  the  near  infra-red,  but 
obtained  results  only  for  the  intense  absorption  band  at  3  ju. 

The  absorption  bands  of  water  at  3  (JL  and  6  n  are  so  intense  that 
extremely  thin  layers  are  necessary  for  their  study,  so  that  a  quantitative 
comparison  of  the  absorption  of  solutions  and  water  would  be  very 
difficult.  The  other  four  absorption  bands  mentioned  above  are  such 
that  measurable  thicknesses  can  be  used  and  an  accurate  comparison 
made.  It  was  the  purpose  of  this  investigation  to  study  the  effect  of 
various  dissolved  substances  on  the  absorption  of  water  throughout  the 
spectral  region  containing  these  four  absorption  bands. 

APPARATUS  AND  EXPERIMENTAL  PROCEDURE. 

It  was  necessary  to  obtain  a  spectrometer  system  free  from  appreciable 
stray  radiation.  Since  no  single  spectrometer  was  at  hand  in  which  the 
stray  radiation  was  negligible,  two  spectrometers  were  used,  one  serving 
as  a  monochromatic  illuminator  for  the  slit  of  the  second.  As  measure- 
ments were  to  be  made  to  2.3  /*  only,  spectrometers  with  glass  lenses  and 
prisms  were  used.  The  spectrometers  were  of  the  constant  deviation 
type  and  arranged  as  illustrated  in  Fig.  I. 

The  source  of  radiation,  Sf,  is  a  series  street  lighting  lamp  with  a 
single  spiral  filament  set  vertically.  It  was  run  on  storage  batteries  to 
insure  constant  current.  LI  is  a  lens  so  placed  as  to  render  the  light 
from  S'  parallel.  Diaphragms  D1  and  D2  were  placed  in  the  beam  to 

1  PHYS.  REV.,  3.  P-  47  (1914). 


488 


J.   R.   COLLINS. 


[SECOND 

[SERIES. 


limit  this  beam,  so  that  its  cross-section  was  smaller  than  that  of  the  cells 
under  investigation.  These  cells  were  mounted  on  a  slider  P' .  This 
slider  could  be  moved  by  the  observer  by  means  of  cords,  so  that  either 
cell  or  an  opaque  screen  was  placed  in  the  beam.  L2  is  a  lens  which 
formed  an  image  of  the  lamp  filament  on  the  slit  Si  of  the  first  spec- 


Fig.  1. 

trometer.  L3,  PI,  and  L4  are  the  collimator  lens,  prism,  and  telescope 
lens  respectively,  of  this  spectrometer,  the  spectrum  being  focused  in  the 
plane  of  the  slit  S%  of  the  second  spectrometer,  of  which  Ls,  PI,  and  Le 
are  the  collimator  lens,  prism  and  telescope  lens  respectively.  The 
spectrum  is  focused  in  the  plane  of  the  slit  53,  behind  which  a  Coblentz 
thermopile  is  mounted.  The  thermopile  is  connected  to  a  Coblentz 
ironclad  galvanometer  which  was  read  by  means  of  a  telescope  and  scale. 

The  second  spectrometer  was  used  to  indicate  the  wave-length  of  the 
radiation  incident  on  the  thermopile.  The  scale  on  the  drum  was 
divided  uniformly,  the  smallest  divisions  corresponding  to  a  range  of 
about  0.3  juju  in  the  visible  spectrum.  This  spectrometer  was  calibrated 
by  observing  the  drum  readings  corresponding  to  the  positions  of  the 
emission  lines  of  sodium,  lithium,  potassium  and  mercury.  The  lines 
of  the  first  three  metals  were  obtained  by  volatilizing  their  salts  in  the 
open  arc,  and  a  quartz  mercury  arc  was  used  for  the  mercury  lines.  A 
considerable  number  of  lines  were  easy  to  locate  and  the  location  of  the 
lines  in  the  visible  region  could  be  checked  visually.  The  wave-lengths 
of  these  lines  were  obtained  from  Paschen's  data.1 

The  cells  to  contain  the  liquid  under  test  consisted  of  two  pieces  of  plane 
parallel  quartz  or  pyrex  glass  between  which,  for  the  thicker  cells,  was 
placed  a  glass  tube  with  plane  parallel  ends  filled  with  the  liquid.  For 
the  thinner  cells  washers  of  thin  glass  or  celluloid  were  used.  Since  the 
two  spectrometers  were  used  in  series  it  was  necessary  that  the  cells 

1  Ann.  d.  Physik.,  27,  p.  537  (1908). 


VoL.^XX.J  INFRA-RED   ABSORPTION   OF    WATER.  489 

have  plane  parallel  ends  and  that  the  liquid  be  of  uniform  thickness  over 
the  entire  cell.  Otherwise  a  change  in  the  direction  of  the  beam  of  radia- 
tion resulted  in  a  change  in  the  amount  of  radiation  reaching  the  thermo- 
pile even  with  no  absorption.  Thus  two  pieces  of  transparent  quartz, 
one  plane  parallel  and  the  other  slightly  wedge  shaped,  appeared  to  have 
different  transmissions  when  placed  alternately  in  the  beam  of  radiation. 
This  effect  was  not  noticeable  if  the  variations  in  thickness  were  not  more 
than  o.ooi  cm.  in  a  plate  of  3  cm.  diameter. 

The  coefficient  of  absorption  of  water  was  determined  throughout 
the  spectral  region  0.8  M  to  2.3  M  by  finding  the  absorption  of  known 
thicknesses.  In  order  to  avoid  the  error  due  to  reflection  at  the  surfaces 
of  the  cells,  two  cells  of  different  thicknesses  were  mounted  on  the  slider 
P'  (Fig.  i)  and  the  deflections  of  the  galvanometer  observed  when  the 
cells  were  placed  in  turn  in  the  beam  entering  the  spectrometer  system. 
The  ratio  of  these  deflections  gives  the  fractional  transmission  of  a  layer 
of  water  equal  in  thickness  to  the  difference  of  thicknesses  of  the  two  cells. 

The  fractional  transmission  of  the  solutions  was  obtained  by  mounting 
on  the  slider  P'  two  cells  of  approximately  the  same  thickness,  one 
containing  the  solution  and  the  other  containing  pure  water.  The 
galvanometer  deflections  were  observed  when  these  cells  were  placed  in 
turn  in  the  beam  of  radiation.  The  ratio  of  their  transmissions  were 
thus  obtained.  The  fractional  transmission  of  the  water  could  be  com- 
puted from  the  known  coefficients  of  absorption  and  then  the  fractional 
transmission  of  the  solution  calculated  from  that  of  water  and  their 
ratio.  There  are  two  advantages  of  comparing  the  transmission  of  the 
solution  to  that  of  water  instead  of  determining  the  fractional  trans- 
mission of  the  solution  by  using  two  cells  of  solution.  First,  it  is  the 
object  of  the  experiments  to  compare  the  transmission  of  the  solution 
with  that  of  water  and  this  method  does  that  with  one  set  of  observations 
instead  of  two.  Second,  a  small  error  in  the  wave-length  setting  of  the 
spectrometer  on  the  side  of  an  absorption  band  will  cause  a  large  change 
in  the  fractional  transmission  observed  if  the  fractional  transmission  of 
the  solution  is  determined  directly;  the  ratio  of  the  transmission  of  the 
solution  to  that  of  water  does  not  change  so  rapidly  as  we  proceed  along 
the  spectrum  as  does  the  fractional  transmission,  hence  a  small  error  in 
wave-length  setting  does  not  affect  the  results  so  much. 

Since  the  four  absorption  bands  of  water  have  different  intensities,  the 
cells  to  contain  the  liquid  were  of  different  thicknesses  for  each  of  the 
four  parts  of  the  spectral  region  studied.  These  thicknesses  were:  about 
2  cm.  for  the  range  0.8  /*  to  i.i  /*;  about  I  cm.  for  the  range  i.i  M  to  1.3  JJL; 
about  0.04  cm.  for  the  range  1.3  /JL  to  1.7  M  ;  about  0.025  cm-  f°r  the  range 
1.7  M  to  2.3  M. 


490 


J.   R.    COLLINS. 


[SECOND 

[SERIES. 


EXPERIMENTAL  RESULTS. 

I.  The  Absorption  Spectrum  of  Water  from  0.8  ju  to  2.3  /JL. 
The  fractional  transmission  of  water  for  known  thicknesses  was  care- 
fully determined  throughout  the  spectral  range  studied.  The  agreement 
of  the  values  of  absorption  coefficients  calculated  from  the  observed 
transmission  of  widely  different  thicknesses  of  water  indicate  that  the 
apparatus  was  free  from  appreciable  stray  radiation.  A  comparison  with 
the  values  obtained  by  Aschkinass1  and  also  those  calculated  from  the 
data  of  Jones  and  his  co-workers2  can  be  made  only  at  the  positions  of 
maximum  absorption  since  the  wave-length  calibration  of  the  prism  used 
by  Aschkinass  was  apparently  not  correct.3  Table  I.  gives  the  values 
of  the  absorption  coefficients  at  these  positions. 

TABLE  I. 


Position  of 

Coefficients  of  Absorption. 

Maximum  Absorption. 

Aschkinass. 

Jones  et  al. 

Collins. 

.97  ju.  •  •  

.416 

.446 

.448 

1.20  /i  

1.221 

1.297 

1  220 

1  44  At 

38  4 

29  4 

2  00  M 

123  2 

103 

The  agreement  at  the  first  two  absorption  bands  at  .97  //,  and  i.2O/z 
is  remarkably  good.  At  the  other  two  bands,  however,  the  agreement  is 
not  good.  The  thicknesses  used  by  Aschkinass  were  much  smaller  than 
those  used  in  the  present  investigation  and  could  not  be  measured  so 
accurately  by  the  method  used  by  him.  This  may  be  a  possible  explana- 
tion of  the  differences  shown  in  Table  I. 

As  may  be  seen  from  Table  II.,  the  wave-lengths  of  maximum  absorp- 
tion as  determined  in  the  present  investigation  do  not  agree  with  those 
obtained  by  some  earlier  observers.  The  care  taken  in  eliminating  stray 
radiation  in  the  present  investigation  may  account  for  some  of  the 
differences  in  the  positions  of  the  absorption  bands  as  compared  with  the 
results  of  others. 

II.  The  Absorption  Spectra  of  Aqueous  Solutions. 

In  order  to  compare  the  absorption  spectra  of  the  solutions  with  that 
of  water  the  amount  of  water  in  each  solution  was  computed  from  a 

1  Ann.  d.  Physik,  55,  p.  401  (1895). 

2  Loc.  cit. 

3  See  Coblentz,  Carnegie  Inst.  of  Wash.  Pub.  65  (1906). 


VOL.  XX.] 
No.  5. 


INFRA-RED   ABSORPTION   OF    WATER. 


491 


knowledge  of  the  concentration  and  density  of  the  solution.     Then  two 
curves  were  plotted  on  the  same  axes:    (i)  the  per  cent,  transmission  of 

TABLE  II. 


Observer. 

Date. 

Kind  of  Prism. 

Position  of  Maximum  Absorption. 

Paschen  
Aschkinass  .... 
Donath 

1894 
1895 
1896 
1910 
1921 
1921 

Fluorspar 
Flint  Glass 
Flint  Glass 
Fluorite 
Rocksalt 
Flint  Glass 

1.00/z 
.97 

1.24  M 

1.51  M 
1.50 
1.45 
1.48 

1.48 
1.44 

2.05  /» 
1.96 
1.96 
1.95 
1.98 
2.00 

Coblentz  

Collins  

1.20 

the  solution  as  a  function  of  the  wave-length  and  (2)  the  per  cent,  trans- 
mission of  a  layer  of  water  equivalent  to  that  present  in  the  solution. 
Four  curves  are  plotted  for  each  substance  as  different  thicknesses  had 
to  be  used  for  different  parts  of  the  spectral  region  studied.  The  thick- 
nesses of  layers  used  were  in  all  cases  those  indicated  in  Fig.  3.  If  the 
dissolved  substance  has  an  effect  on  the  absorption  of  the  water  or  has 
an  absorption  of  its  own  the  two  curves  would  not  coincide.  If  the  trans- 
mission curve  for  the  solution  lies  above  that  for  water,  the  dissolved 
substance  has  had  an  effect  on  the  absorption  of  the  water.  If,  however, 
the  curve  for  the  solution  lies  below  that  for  the  water,  there  are  two 
possible  explanations  and  in  general  it  is  impossible  to  separate  them. 
It  seems  unlikely  that  the  absorption  bands  for  the  dissolved  substance 
would  lie  exactly  at  the  same  position  and  have  the  same  shape  as  the 
water  absorption  band,  so  that  the  transmission  curves  for  the  solutions 
should  be  distorted  from  those  of  water  if  the  dissolved  substances  have 
absorption  bands  in  the  region  studied.  An  example  of  this  is  shown  in 
Fig.  2. 


\ 


V 


'Wfltar  __ 


Wove    Lengths    in  M/crons 

Fig.  2. 
5Ar  solution  of  NaOH.     200  grams  per  liter.     Density  1.186  gr./c.c. 


492 


J.   R.   COLLINS. 


[SECOND 
[SERIES. 


Solutions  of  NaOH,  KOH  and  LiOH  were  studied  and  curves  for  NaOH 
only  are  shown  as  they  are  typical  of  the  results  for  all  three  substances. 
Grantham  has  already  studied  solutions  of  these  substances  in  the 
region  of  the  absorption  at  1 .44  /*  and  showed  that  all  of  them  rendered 
the  water  less  absorbing  in  that  band  in  spite  of  an  absorption  band  of 
the  substances  themselves  which  overlaps  that  of  water.  Curves  A  and 
B  of  Fig.  2  show  the  effect  of  NaOH  on  water  in  the  bands  at  .97  JJL  and 
1. 20  ju-  At  i. 20  n  there  is  a  decided  increase  in  the  transmission  but  at 
the  other  band  there  is  a  decrease  in  transmission  on  one  side  of  the  band. 
From  the  distorted  shape  of  the  solution  curve  it  was  suspected  that  the 
hydroxides  had  absorption  bands  near  .96/1,  i.i  /*  and  1.27  /JL.  Concen- 
trated solutions  were  made  of  KOH  in  absolute  methyl  alcohol  and 
absolute  ethyl  alcohol  and  the  transmission  curves  determined.  Absorp- 
tion bands  were  found  at  the  wave-lengths  .95  JJL  and  i.i  /*  and  1.26  ju. 
It  seems  safe  to  say  then  that  these  three  alkaline  hydroxides  make  water 
more  transparent  in  all  three  of  the  absorption  bands  shown  in  Fig.  2. 

Figures  3  to  15  show  the  results  obtained  for  several  salts,  all  except 
the  last  three  being  hydra  ting  substances.  A12(SO4)3?  ZnSO4  and  the 


V 


—Water 
—  Soli/ti 


-if  3 


Wave    Lengths   in  Microns 

Fig.  3. 

A12(SO4)3.     259.3  gr./liter.     Density,  1.2200  gr./c.c. 

Curve  A,  2.090  cm.  layer  of  solution.     Curve  B,  .997  cm.  layer  of  solution. 
Curve  C,  .025  cm.  layer  of  solution.     Curve  D,  .0079  cm.  layer  of  solution. 

alkaline  hydroxides  decrease  the  absorption  of  water  in  all  four  absorp- 
tion bands  studied.     All  other  substances  studied  decreased  the  absorp- 


VOL.  XX/1 
No.  s.       J 


INFRA-RED  ABSORPTION  OF    WATER. 


493 


tion  in  the  absorption  band  at  1 .44  JJL,  increased  the  absorption  in  the 
absorption  bands  at  .97  ^  and  1.20  //.  The  absorption  band  at  2.00  /*  is 
not  affected  in  the  same  way  by  all  substances  but  in  most  cases  where 


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Aids.     326.0  gr./liter.     Density,  1.2133  gr./c.c. 

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Fig.  6. 
CaCh.     557.7  gr./liter.     Density,  1.3950  gr./c.c. 

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SrCla.     296.0  grams/liter.     Density  1.2445  gr./c.c. 


494 


/.   R.   COLLINS. 


[SECOND 
LSERIES. 


the  absorption  is  increased  the  irregularity  of  the  curve  for  the  solution 

indicates  that  the  dissolved  substance  had  absorption  bands  in  this  region. 

No  definite  conclusions  can  be  drawn  as  to  shifts  in  the  positions  of 


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VOL.  XX.l 
Ko.5. 


TNFRA-RED   ABSORPTION   OF    WATER. 


495 


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Fig.  12. 
Na2S2O3.     524.9  gr./liter.     Density,  1.361  gr./liter. 

Fig.  13. 
NaCl.     319.8  gr./liter.     Density,  1.2020  gr./c.c. 

Fig.  14. 
AgNOs.     1413.0  gr./liter.     Density,  2.1180  gr./c.c. 


Fig.  15. 
NH4NO3.     743.0  gr./liter.     Density,  1.2700  gr./c.c. 


496 


J.   R.   COLLINS. 


[SECOND 

LSERIES. 


an  absorption  band  of  the  dissolved  substance  near  that  of  the  water 
might  cause  such  a  change  in  the  shape  of  the  curve. 


Fig.  16. 

DISCUSSION. 

Schaeffer,  Paulus,  and  Jones  explained  their  results  on  the  basis  of  the 
solvate  theory.  The  dissolved  substances  which  form  loose  combinations 
with  some  of  the  water  in  which  they  are  dissolved  render  this  part  of  the 
water  less  absorbing.  Non-hydrating  substances  should  have  no  effect 
on  the  absorption  of  the  water.  Their  explanation  did  not  fit  all  the 
data  which  they  published  and  does  not  agree  with  all  the  results  shown 
here.  A12(SO4)3,  ZnSO4,  and  the  alkaline  hydroxides  make  the  water 
less  absorbing  at  all  the  absorption  bands,  but  all  the  remaining  hydrating 
salts  studied  cause  a  greater  absorption  at  some  of  the  absorption  bands. 
Also  the  three  non-hydrating  salts,  NaCl,  NH4NO3,  and  AgNO3  all 
caused  a  decrease  in  absorption  by  water  at  one  absorption  band. 

Livens1  has  deduced  an  explanation  of  an  effect  of  a  dissolved  substance 
on  the  absorption  of  the  solvent  on  the  basis  of  the  Lorentz  electron 
theory.  If  the  solute  has  absorption  bands  at  wave-lengths  much  longer 
than  that  of  the  solvent,  the  solvent  should  absorb  less  than  it  would  if 
the  solute  were  absent  and  the  position  of  maximum  absorption  should 
be  shifted  toward  the  shorter  wave-lengths.  If  the  solute  has  absorption 
bands  at  wave-lengths  shorter  than  that  of  the  solvent,  the  solvent 
should  be  more  absorbing  in  the  presence  of  the  solute  and  the  position 

1  Phys.  Zeitschr.,  14,  p.  660  (1913). 


VOL.^XX.J  INFRA-RED   ABSORPTION   OF    WATER.  497 

of  maximum  absorption  should  be  shifted  toward  the  longer  wave- 
lengths. This  explanation  would  seem  to  lead  to  the  conclusion  that  all 
the  absorption  bands  of  water  here  studied  should  be  affected  alike,  as 
they  lie  close  together  and  thus  have  the  same  relation  to  the  absorption 
bands  of  the  dissolved  substance. 

Neither  of  these  explanations  fits  the  experimentally  determined  facts 
completely.  With  the  exceptions  of  A12(SO4)3,  ZnSO4  and  the  alkaline 
hydroxides,  the  solutes  used  caused  an  increase  in  the  absorption  of  the 
water  at  the  absorption  bands  located  at  .97  ju  and  1 .20  /*,  and  a  decrease 
in  the  absorption  of  the  water  at  the  band  located  at  1.44  IJL.  From  an 
examination  of  the  curves  it  seems  likely  that  the  dissolved  substances 
decrease  the  absorption  of  the  water  in  the  band  at  2.00  ju,  although  in 
many  cases  this  effect  is  apparently  masked  by  the  presence  of  absorption 
bands  of  the  dissolved  substance. 

With  the  above  exceptions  then  it  may  be  said  that  the  effect  of  dis- 
solved substances  is  to  increase  the  absorption  of  the  water  at  two  absorp- 
tion bands  (.97  ^  and  1.20  IJL)  and  to  decrease  the  absorption  of  the  water 
at  the  other  two  bands  (1.44/4  and  2.00  IJL).  A  parallel  to  this  is  found 
by  comparing  the  absorption  of  water  vapor  and  liquid  water  for  layers 
having  equal  number  of  molecules.  Fig.  16  shows  the  absorption  spectra 
of  equivalent  thicknesses  of  water  vapor  and  liquid  water.  The  water- 
vapor  curve  is  taken  from  the  results  of  Hettner.1  There  is  an  absorption 
band  in  the  vapor  curve  for  each  band  in  the  liquid  curve,  but  displaced 
toward  the  shorter  wave-lengths.  The  absorption  bands  of  water  at 
.97  IJL  and  1. 20  fjL  are  less  intense  than  the  corresponding  bands  for  the 
vapor,  while  the  bands  at  1.44^  and  2.00  fj.  are  more  intense  than  the 
corresponding  vapor  bands.  This  fact  suggests  that  there  may  be  some 
connection  between  the  effect  of  dissolved  substances  on  the  absorption 
of  water  and  the  effect  of  changing  from  vapor  to  liquid. 

The  fact  that  all  the  absorption  bands  of  liquid  water  are  present  when 
the  water  is  converted  into  vapor  indicate  that  these  absorption  bands 
are  due  to  the  molecules  and  not  to  aggregations  of  molecules.  Hence  it 
seems  that  the  only  way  in  which  the  absorption  could  be  changed  to  a 
great  extent  would  be  by  a  change  in  the  number  of  absorbing  molecules 
present.  If  there  are  different  kinds  of  water  molecules  there  is  a  possi- 
bility of  their  relative  number  being  changed  by  the  presence  of  a  dis- 
solved substance,  by  polymerization  of  the  water,  by  change  of  tempera- 
ture, etc.  According  to  Langmuir's2  model  of  the  water  molecule  there 
may  be  four  kinds  of  water  molecules  as  the  hydrogen  nuclei  may  be 

1  Ann.  d.  Physik.,  55,  p.  476  (1919). 

2  Jour.  Amer.  Chem.  Soc.,  41,  p.  893  (1919). 


498  /.  R.  COLLINS.  [§ER?ESD 

arranged  in  four  different  relative  positions  about  the  oxygen  nucleus. 
These  different  molecules  would  probably  have  different  modes  of  vibra- 
tion which  would  give  rise  to  absorption  bands  of  different  frequencies. 
The  relative  intensities  of  these  absorption  bands  would  depend  in  part 
on  the  relative  number  of  each  kind  of  molecule  present  in  the  water. 
Some  such  explanation  together  with  the  fact  that  some  dissolved  sub- 
stances form  loose  combinations  with  the  water  in  which  they  are 
dissolved  may  be  able  to  fit  the  experimental  results  when  more  data  are 
available.  At  present  no  simple  explanation  fits  all  the  known  results. 
The  writer  wishes  to  thank  Professor  R.  C.  Gibbs,  under  whose 
direction  the  investigation  was  carried  out,  for  his  encouragement  and 
helpful  advice  during  its  progress. 
CORNELL  UNIVERSITY. 


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BERKELEY 

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